1. Field of the Invention
The present invention relates generally to implantable medical devices and more particularly relates to implantable medical devices for stimulation of the central and peripheral nervous systems.
2. Description of the Prior Art
Chronically implantable stimulators for the brain and spinal cord have been in use for some time. Originally these were used to treat chronic intractable pain. Clinically favorable results were reported in a number of publications including "Long Term Follow Up of Dorsal Cord Stimulation for Chronic Pain Syndrome After Multiple Lumbar Operations", Applied Neurophysiology, Volume 45, pages 201-204, 1982 by J. Siegfried and Y. Lazorthes and "Spinal Epidural Neurostimulation for Treatment of Acute and Chronic Intractable Pain: Initial and Long Term Results", Neurosurgery, Volume 5, pages 344-348, 1979, by R. R. Richardson, et al.
Subsequently, additional medical applications were reported including treatment of peripheral vascular disease as seen in "Spinal Cord Stimulation in Peripheral Vascular Disease" Proceedings on Functional Electrostimulation, 1983, by E. H. Sedgwick, L. S. Illis, and R. C. Tallis. Research continues in the possible treatment of angina and other disorders by spinal cord stimulation.
The earliest practical spinal cord stimulators were passive devices which picked up RF energy transmitted across the skin and applied it via electrodes implanted near the spinal cord. Many medical applications are yet best approached with such a device. The PISCES.RTM. family of spinal cord stimulation systems made available to the medical community by Medtronic, Inc. of Minneapolis, Minn., assignee of the present invention, are examples of passive spinal cord stimulation systems.
A later development in neurological stimulation is the chronically implantable active stimulation system. These consist of a battery operated pulse generator which is electrically coupled to the spinal cord by insulated leads coupled to electrodes normally implanted in the epidural space. The Medtronic.RTM. Itrel.RTM. implantable neurostimulator is an example of an active device. The Itrel II.RTM. implantable pulse generator manufactured Medtronic, Inc. has advanced programmable features permitting mode changes by transcutaneous RF telemetry. These mode changes include modifying the stimulation intensity.
Clinical results have been reported that suggest it is appropriate to lower the stimulation intensity whenever a patient is recumbent. The lower stimulation intensity is adequate for medical efficacy of the treated condition in the supine patient and undesirable side effects are reduced. See "Electrical Stimulation of the Spinal Cord. The Phenomenon of Changing Paresthesias", by G. H. Spincemaille and C. H. A. Wittens. Similarly, increasing intensity is preferred in the erect patient to achieve optimal effectiveness of stimulation.